The present invention relates to the field of hydraulics. More particularly, this invention relates to variable displacement pumps in which non-feedback proportional control of the pump""s displacement is desired. For a given operating condition, a non-feedback proportional control provides an output flow proportional to an input hydraulic signal (pressure) or electronic signal (current). The present invention is particularly useful when an electronic signal is available.
Various non-feedback proportional electronic (hereinafter NFPE) control assemblies are known and have become increasingly popular for axial piston variable displacement pumps used to propel various types of off-road vehicles, such as wheel loaders, aerial lifts, and various material handlers. One known NFPE control assembly makes use of a first NFPE control valve with a first proportional solenoid for controlling pump output in one direction and a second NFPE control valve with a second proportional solenoid for controlling pump output in the opposite direction. This conventional design is shown in FIG. 1.
Generally, vehicle operators want the pump to exhibit the same operating characteristics in forward and reverse. Any variation in operating characteristics (i.e., threshold, slope, etc.) between the two control valves and their respective proportional solenoids adds to the forward to reverse asymmetry exhibited by the pump. These variations exhibit themselves in undesirable variations in neutral threshold, full stroke and flow forward/current slope values for the pump being controlled. There is a need for a new type of NFPE control assembly that provides built-in symmetry or similarity between its forward and reverse command signals.
Therefore, a primary objective of the present invention is the provision of a NFPE control assembly that has built-in forward-to-reverse symmetry.
Another objective of this invention is the provision of a NFPE control assembly that avoids the problem of NFPE control valve-to-valve variation within the control assembly.
Another objective of this invention is the provision of a NFPE control system that utilizes a single NFPE control valve (proportional solenoid) to control the magnitude of the signal, coupled with a directional control valve to route control flow to the appropriate servo or side of the servo.
Another objective of the present invention is the provision of a NFPE control assembly that is less costly than existing arrangements.
Another objective of the present invention is the provision of a hydraulic circuit that includes a variable displacement hydraulic pump and a NFPE control assembly with symmetrical forward and reverse outputs.
These and other objectives will be apparent to one skilled in the art from the drawings, as well as from the description and claims that follow.
The present invention relates to a hydraulic circuit that includes a variable displacement hydraulic pump and a control assembly for the pump. The control assembly is a two-stage non-feedback proportional control assembly that is operatively associated with the pump. The first stage of the control assembly includes a single non-feedback proportional control valve that generates a displacement magnitude hydraulic signal that is proportional to an input signal. The second stage of the control assembly includes a directional control valve located downstream of the proportional control valve for conditioning and routing the displacement magnitude signal to the pump with an appropriate directional indicator.